Generally, microscopic spectrometers use refractive optics. However, refractive optics are not adequate for certain wavelength ranges; for example, soft x-rays, ultraviolet rays, or infrared rays. In this wavelength range, microscopic spectrometers utilize a Schwarzshild-type Cassegrain objective (hereinafter referred to as a "Cassegrain objective"). The Cassegrain objective includes two opposing spherical mirrors, which are used together to create an objective. The operation of the Cassegrain objective is discussed further below.
When spectrometrically measuring an electromagnetic wave within the wavelength ranges where refractive optics are inadequate, a mask is placed in the image plane of the Cassegrain objective to limit the area of the sample to be measured and avoid the mingling of optical information about the various areas on the sample. This is particularly important when measuring across the frequency spectrum. Since a detecting element cannot be placed in the image plane, the detecting element is placed behind the image plane to obtain information about the part of the image plane which is not masked.
However, microscopic spectrometers using Cassegrain objectives also use a Cassegrain mirror system in the relay optical system. In infrared microscopic spectrometry, a Hg-Cd-Te crystal is used as a detecting element. Since the sensitivity of the crystal is inversely proportional to the volume of the crystal, the crystal is miniaturized as much as possible. Therefore, the relay optical system is configured to project the image produced by the Cassegrain objective into a small contracted area.
FIG. 6 shows a conventional microscopic spectrometer. Therein, a Cassegrain objective 1 includes a concave main mirror 4 having a central opening 2. The mirrored surface of the main mirror 4 faces the sample 3 and a secondary mirror 5. The main mirror 4 reflects an image of the sample 3 onto the secondary mirror 5. The secondary mirror 5 has a convex mirrored surface facing the concave mirrored surface of the main mirror 4.
A relay optical system 6 also includes an inverse placed Cassegrain objective system (hereinafter referred to as "Cassegrain relay optics"). The Cassegrain relay optics 6 includes a concave main mirror 9 with a central opening 7. The mirrored surface of the main mirror 9 faces away from the Cassegrain objective 1, thereby providing a reflective surface between the secondary mirror 10 and a detecting element 8. The secondary mirror 10 has a convex mirrored surface and is positioned on the opposite side of the main mirror 9 from the Cassegrain objective 1. In this configuration, each of the optical elements is operatively positioned along an optical axis 11, allowing interaction with an image plane -2 which is created by the Cassegrain objective 1.
In the configuration illustrated in FIG. 6, the secondary mirror 5 serves as surface inside the pupil of the Cassegrain objective 1. The central opening 2 is the pupil of the Cassegrain objective 1. The secondary mirror 5 obstructs a portion of the sample 3 from being viewed by the main mirror 4. The light information reflected by the secondary mirror 5 is hollow in that it lacks information regarding a central portion of the sample 3.
FIG. 7(a) illustrates the light information reflected from the pupil surface of the secondary mirror 5. Therein, A represents the diameter of the pupil surface of the secondary mirror 5, and B designates the portion of the light information which has been previously obstructed by the secondary mirror 5.
FIG. 7(b) is a likewise illustration of the optical throughput of the Cassegrain relay optics 6. Therein, C represents the diameter of the pupil of the secondary mirror 1, and D represents the information which is obstructed by the secondary mirror 10 from reaching the detecting element 8.
Therefore, the efficient transmission of light from the sample 3 to the detecting element 8 through the configuration illustrated in FIG. 6 requires that the pupils A and C coincide in size and shading coefficient. Consequently, in the best configuration, A =C and B =D.
This rule for the efficient transmission of light in the optical configuration shown in FIG. 6 only holds true when the sample 3 is disposed on the optical axis 1 1. A sample which is slightly displaced from the optical axis 11 is illustrated by reference numeral 3'. In this configuration, light transmitted from the sample 3' cannot be efficiently focused onto the detecting element 8. Furthermore, when the Cassegrain systems 1, 6 are different in magnification and numerical aperture, or if the pupils are different in size and shading coefficient, even a sample 3 disposed on the optical axis 11 will not produce an efficient transmission of light to the detector 8.
FIG. 7 illustrates the various pupil configurations and transmissions of the optical configuration illustrated in FIG. 6. As explained above, FIGS. 7(a) and 7(b) show the pupil size and transmissions for the Cassegrain objective 1 and the Cassegrain relay optics 6, respectively. As discussed above, when A =C and B =D and the sample 3 is disposed on the optical axis 11, the projection of the pupil of the Cassegrain objective 1 onto the pupil of the Cassegrain relay optics 6 produces the configuration shown in FIG. 7(c), and the effective beam which is transmitted to the detecting element 8 is shown in FIG. 7(d).
In FIG. 7(d), the lattice portion E illustrates the light information which impinges upon the detecting element 8, and the nonlatticed portion F illustrates the information from the sample 3 which is obstructed by the secondary mirrors 5 and 10. The nonlatticed portion F is minimized because the secondary mirror 10 introduces no additional obstruction to the obstruction caused by the secondary mirror 5.
However, when the sample 3' is introduced configuration shown in FIG. 6, the transmission of the pupils of the Cassegrain objective 1 and the Cassegrain relay optics 6 are shown overlapped in FIG. 7(e), and the effective beam transmitted to the detecting element 8 is shown in FIG. 7(f) as the lattice portion E'. The lattice portion E' is considerably less in area than the lattice portion E shown in FIG. 7(d). Furthermore, the nonlatticed portion F' shows the obstructed portion which is considerably greater in area than the obstructed portion F shown in FIG. 7(d). Light from the sample 3' cannot be efficiently transmitted to the detecting element 8 because the sample 3' is not present on the optical axis 11.
A paraboloid mirror or an ellipsoid mirror might be used in the Cassegrain relay optics 6 discussed above to make the pupils more adaptable when the sample 3' is displaced from the optical axis 11. However, since the relay optics 6 transmits light from the Cassegrain objective 1 to the detecting element 8, the numerical aperture of the relay optics 6 must be at least the same as the objective 1. On the other hand, it is extremely advantageous to use a smaller detecting element 8, as described above, so that the relay optics 6 contracts the light transmitted into a small area.
For example, if (1) the diameter of the detecting element 8 is 250 microns, (2) the diameter of the maximum measuring field of view is 250 microns, (3) the magnification of Cassegrain objective 1 is 15, and (4) the numerical aperture on the sample side is 0.3, then the numerical aperture on the image side of the Cassegrain objective 1 becomes 0.02, and the diameter of the image becomes 3.75 millimeters. If the Cassegrain objective 1 is to transmit light to an ellipsoid mirror 14 for focusing onto the detecting element 8, as shown in FIG. 8, the contraction factor is 1/15 when the short focal distance of the eccentric ellipsoid mirror 14 is 50 millimeters, and the long focal distance becomes 750 millimeters.
Furthermore, if an angle between the optical axis 11 and the highest point of information being transmitted by the secondary mirror 5 of the Cassegrain objective 1 to the image plane 12 is 0.5 degrees, then the distance of that information from the optical axis 11 at the ellipsoid mirror 14 becomes 6.5 millimeters. For the geometry shown in FIG. 8, this corresponds to a 125-micron height on the sample 3. In that case, the numerical aperture becomes 0.02, and the entire diameter of 30 millimeters. Thus, the effective diameter required for the ellipsoid mirror 14 becomes 43 millimeters or more, making the apparatus extremely large-sized. Since the long focal distance of the ellipsoid mirror 14 increases as the contraction factor of the ellipsoid mirror 14 is reduced, the overall apparatus becomes extremely large and cumbersome.
When the microscope is used in the infrared spectrum, an additional disadvantage stems from the absorption of transmitted light between the optical elements by carbonic acid gas and steam present along the optical path. Thus, the use of an ellipsoid mirror 14 is additionally not preferable when used in infrared microscopes.
The above-referenced discussion is additionally applicable to paraboloid mirrors used in place of the ellipsoid mirror 14 because two paraboloid mirrors would be used having focal distances of 750 millimeters and 50 millimeters, respectively.